Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.
Assume a BST is defined as follows:
<li>The left subtree of a node contains only nodes with keys <b>less than or equal to</b> the node's key.</li>
<li>The right subtree of a node contains only nodes with keys <b>greater than or equal to</b> the node's key.</li>
<li>Both the left and right subtrees must also be binary search trees.</li>
For example:
Given BST [1,null,2,2],
1
\
2
/
2
return [2].
Note: If a tree has more than one mode, you can return them in any order.
Follow up: Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).